Confinement of Ultrasmall Cu/ZnOx Nanoparticles in Metal-Organic Frameworks for Selective Methanol Synthesis from Catalytic Hydrogenation of CO2

Article abstract of DOI:10.1021/jacs.7b00058

The interfaces of Cu/ZnO and Cu/ZrO₂ play vital roles in the hydrogenation of CO₂ to methanol by these composite catalysts. Surface structural reorganization and particle growth during catalysis deleteriously reduce these active interfaces, diminishing both catalytic activities and MeOH selectivities. Here we report the use of preassembled bpy and Zr₆(μ₃-O)₄(μ₃-OH)₄ sites in UiO-bpy metal-organic frameworks (MOFs) to anchor ultrasmall Cu/ZnO_x nanoparticles, thus preventing the agglomeration of Cu NPs and phase separation between Cu and ZnO_x in MOF-cavity-confined Cu/ZnO_x nanoparticles. The resultant Cu/ZnO_x@MOF catalysts show very high activity with a space-time yield of up to 2.59 g_MeOH kg_Cu^-1 h^-1, 100% selectivity for CO₂ hydrogenation to methanol, and high stability over 100 h. These new types of strong metal-support interactions between metallic nanoparticles and organic chelates/metal-oxo clusters offer new opportunities in fine-tuning catalytic activities and selectivities of metal nanoparticles@MOFs.

Full text of DOI:10.1021/jacs.7b00058

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Article
x
Confinement of Ultrasmall Cu/ZnO Nanoparticles in
Metal-Organic Frameworks for Selective Methanol
2
Synthesis from Catalytic Hydrogenation of CO
Bing An, Jingzheng Zhang, Kang Cheng, Pengfei Ji, Cheng Wang, and Wenbin Lin
J. Am. Chem. Soc., Just Accepted Manuscript • Publication Date (Web): 17 Feb 2017
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Confinement of Ultrasmall Cu/ZnOx Nanoparticles in Metal-Organic
Frameworks for Selective Methanol Synthesis from Catalytic Hydro-
genation of CO2
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,†
,†,‡
Bing An,† Jingzheng Zhang,† Kang Cheng,† Pengfei Ji,‡ Cheng Wang* and Wenbin Lin*
†Collaborative Innovation Center of Chemistry for Energy Materials, State Key Laboratory of Physical Chemistry of
Solid Surfaces, Department of Chemistry, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen
361005, P.R. China
‡Department of Chemistry, University of Chicago, 929 East 57th Street, Chicago, Illinois 60637, United States
ABSTRACT: The interfaces of Cu/ZnO and Cu/ZrO2 play vital roles in the hydrogenation of CO2 to methanol by these com-
posite catalysts. Surface structural reorganization and particle growth during catalysis deleteriously reduce these active
interfaces, diminishing both catalytic activities and MeOH selectivities. Here we report the use of pre-assembled bpy and
Zr6(μ3-O)4(μ3-OH)4 sites in UiO-bpy metal-organic frameworks (MOFs) to anchor ultrasmall Cu/ZnOx nanoparticles, thus
preventing the agglomeration of Cu NPs and phase separation between Cu and ZnOx in MOF cavity-confined Cu/ZnOx nano-
particles. The resultant Cu/ZnOx@MOF catalysts show very high activity with a space-time yield of up to 2.59 gMeOH kgCu-1 h-1
and 100% selectivity for CO2 hydrogenation to methanol and high stability over 100 hours. These new types of strong met-
al-support interactions between metallic nanoparticles and organic chelates/metal-oxo clusters offer new opportunities in
fine-tuning catalytic activities and selectivities of metal nanoparticles@MOFs.
10
channels.8d,
We envision that porous metal-organic
■ INTRODUCTION
frameworks (MOFs) can combine both the confinement
effect by the pores and the SMSIs with orthogonal metal-
coordinating sites from the ligands and metal-oxo cluster
secondary building units (SBUs) to provide a novel carrier
for Cu-based CO2 hydrogenation catalysts.
As methanol is a potential clean fuel as well as an
important feedstock to produce commodity chemicals,1 the
catalytic conversion of CO2 to methanol is a highly
desirable chemical process that can also combat the rising
CO2 level in Earth’s atmosphere.2 The ternary
Cu/ZnO/Al2O3 catalyst is industrially used in methanol
systhesis from syngas (CO/H2) and also from CO2
hydrogenation.3 In the latter, CO2 is the direct carbon
source for the methanol production as revealed by isotopic
labeling studies.4 Although the detailed structure of the
active catalytic site is still under debate, there is a
consensus on the critial role of the Cu/ZnOx interface.5
However, Cu nanoparticles (NPs) slowly aggregate and
seperate from ZnOx under reaction conditions, reducing
the Cu/ZnOx interfaces and diminishing the catalytic
activity over time.6 Because the Cu surface can catalyze the
reversed water-gas shift (RWGS) reaction to convert CO2 to
MOFs possess regular pores and cavities for NP
encapsulation and have served as functionalized supports
for highly active catalysts.11 For example, Yaghi and
Somorjai recently reported core-shell structures composed
of 18 nm Cu NPs in the cores and porous MOFs in the shells
for CO2 hydrogenation with high selectivity.8a Organic
coordinating groups on the MOF struts can further
stabilize NPs and tune their surface structures by specific
SMSIs.12 Alternatively, the SBUs in Zr-based MOFs were
recently explored as novel supports for single-site
catalysts.13 We hypothesized that these SBU sites could
also act as oxometalate anchors for metal/metal oxide NPs.
The combination of cavity confinement effects with
specific and tunable SMSIs provides a unique opportunity
to stabilize well-mixed ultrasmall Cu/ZnOx NPs for CO2
hydrogenation.
CO, phase-seperated catalysts exhibit poor selectivity for
7
methanol.3b,
Stabilizing intimately mixed Cu/ZnOx
interfaces is thus crucial to maintaining high catalytic
activity and selectivity.
In this work, we used the UiO-bpy MOF, constructed
from linear 2,2’-bipyridine-5,5’-dicarboxylate (bpydc)
bridging ligands and Zr6(μ3-O)4(μ3-OH)4 SBUs, as a support
for Cu/ZnOx catalysts (Scheme 1). UiO-bpy possesses
Strong metal-support interactions (SMSIs) and
confinement effects have previously been used to stabilize
Cu NPs in catalytic CO2 hydrogenation.8 For example,
zirconia as a carrier interacts more strongly with Cu as
compared to alumina, leading to more dispersed NPs and
enhanced catalyst stability.9 Mesoporous zeolites were also
used to prevent NP growth by confining them in the nano-
o
exceptional thermal stability (>250 C) even under high
humidity.14 Cu2+ ions were first coordinated to the bpy
sites in the MOF by post-synthetic metalation with CuCl2.
Zn2+ ions were then introduced by reacting ZnEt2 with the
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formate at 2873 and 2966 cm-1 in the in situ diffuse-
3-OH sites on the SBUs. Ultrasmall Cu/ZnOx NPs were
Page 2 of 9
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reflectance infrared fourier transform (DRIFT) spectra at
150 C (Figure S3). The open coordination sites on the
SBUs can adsorb and activate CO2 under catalytic
conditions.
generated in situ at a reaction temperature of 250 C in the
presence of H2 as the reductant. These NPs of smaller than
1 nm in diameter reside in the tetrahedral and octahedral
cages confined by the ligands to afford novel
Cu/ZnOx@MOF catalysts with high activity and selectivity
for CO2 hydrogenation to methanol.
o
UiO-bpy was treated with CuCl2 in THF to afford UiO-
bpy-Cu (Figure 1, Figure S4). The coordination of Cu ions
to bpydc was supported by the prescence of a metal-to-
ligand charge transfer (MLCT) band at 470−530 nm and a
Cu2+ d−d transition at ~720 nm in the diffuse reflectance
UV-Vis-NIR spectra of UiO-bpy-Cu (Figure 1b).15 The
Zr3(μ3-OH) sites on UiO-bpy-Cu SBUs were metalated with
ZnEt2 to afford Zr3(μ4-OZnEt) in Zn@UiO-bpy-Cu. The
coordination of Zn ions in Zn@UiO-bpy-Cu was confirmed
by the disappearance of the ν(μ3-OH) vibrational band at
3662 cm-1 in the infrared spectra after metalation with
ZnEt2 (Figure 1d). Every μ4-O-ZnEt moiety is in close
proximity to six carboxylate oxygen atoms from the
ligands that create a unique bowl-shaped coordination
environment for Zn ions.
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Scheme 1. Preparation of CuZn@UiO-bpy via in situ reduction
of post-synthetically metalated UiO-bpy. Cu2+ ions were
coordinated to the bpy groups while Zn2+ ions were attached
to the SBUs.
■ RESULTS
1. Synthesis and characterization of the CuZn@UiO-
bpy catalyst with MOF cavity-confined ultrasmall
Cu/ZnOx nanoparticles
UiO-bpy was synthesized via a solvothermal reaction
between ZrCl4 and H2bpydc in the presence of HCO2H
(Supporting information [SI], Section 3). UiO-bpy is
isostructural to UiO-67, which is constructed from 12-
connected [Zr6(μ3-O)4(μ3-OH)4]12+ SBUs and linear
dicarboxylate ligands in a fcu topology. UiO-bpy contained
9.1 % of formates (based on total carboxylates) on the
Figure 1. Structures and characterization of Cu- and Zn-
functionalized UiO-bpy. a) Idealized structure of the precata-
lyst, Zn@UiO-bpy-Cu, via postsynthetic Cu coordination to bpy
followed by protonolysis of Zr3(μ3-OH) sites on UiO-bpy-Cu
SBUs with ZnEt2; b-e) Diffuse reflectance UV-Vis-NIR spectra
(b), the shift of SPR peaks (c), IR spectra (d) and powder X-ray
diffraction patterns (e) of UiO-bpy and its postsynthetically
metalated derivatives.
1
SBUs as determined by H NMR spectra of the digested
MOFs (Figure S1). Thermogravimetric analysis (TGA) gave
37.7 % weight loss in the temperature range of 400-500 oC
(Figure S2) due to ligand decomposition, suggesting a
bpydc/Zr molar ratio of 5/6 that is consistent with
coordiantion defects on the SBUs. UiO-bpy thus has many
coordination defects and possesses an approximate
formula of Zr6(μ3-O)4(μ3-OH)4(μ1-OH)(H2O)(bpydc)5(HCO2)
(SI, Section 3). The coordinated formate groups
decompose under reaction conditions, as clearly evidenced
by the disappearance of the v(C-H) stretching peak of
Powder
X-ray
diffraction
(PXRD)
studies
demonstrated that all metalated UiO-bpy materials
remained crystalline (Figure 1e). The Cu loading as
determined by inductively coupled plasma-optical
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emission spectrometry (ICP-OES) corresponded to the respectively. The formation of ultrasmall Cu NPs is also
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coordination of 77% of the bpy sites in UiO-bpy whereas
Zn occupied 100% of the Zr3(μ3-OH) sites. An approximate
chemical formula of Zn@UiO-bpy-Cu was thus determined
supported by the high Cu dispersivity of 55.3 %, which is
defined as the percentage of Cu on the NP surface and
measured by N2O oxidation followed with H2 titration.
Cu/ZnOx NPs inside UiO-bpy cannot be identified in TEM
images (Figure 2, S7) or PXRD patterns of CuZn@UiO-bpy
(Figure 1e), likely due to their small sizes and lack of
contrast over Zr6 SBUs. Energy dispersive X-ray
spectroscopy (EDX) mapping indicated uniform
distribution of Cu and Zn throughout the particles of both
Zn@UiO-bpy-Cu (Figure 2d) and CuZn@UiO-bpy (Figure
2e, S8-9, Table S1). The Cu and Zn distributions overlaped
with each other, suggesting that Zn and Cu are well mixed
with no phase separation on the length scale less than 10
nm which is the spatial resolution of EDX.
to
be
Zr6(μ3-O)4(μ3-OZnEt)4(μ1-
OH)(H2O)(bpydc)1.15(bpydc-CuCl2)3.85(HCO2).
Nitrogen sorption experiments gave BET surface
areas of 324.3 m2/g for UiO-bpy-Cu and 89.8 m2/g for
Zn@UiO-bpy-Cu, which are both much smaller than that of
the unmetalated MOFs (3037 m2/g) (Figure S5). The
reduction of BET surface area after metalation is
consistent with high metal loadings and the potential
blocking of pore entrances by the sterically demanding –
CuCl2 and –ZnEt moieties.
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Zn@UiO-bpy-Cu was in situ reduced to form the
To release the Cu/ZnOx NPs from CuZn@UiO-bpy for
direct TEM characterization, we digested the UiO-
framework with a K3PO4 solution in the presence of
polyvinyl pyrrolidone (PVP) to stabilize the ultrasmall NPs
and removed zirconium phosphate solid by centrifugation
at low speed (7500 rpm, 6500 g). Scanning transmission
electron microscopy-high angle annular dark-field (STEM-
HAADF) image of the species in the supernatant clearly
showed nanoparticles of 0.5-2.0 nm in size (Figure 2c,
Figure S10). These ultrasmall NPs contained both Cu and
Zn elements as demonstrated by EDX (Figure S11).
o
CuZn@UiO-bpy catalyst at 250 C and 4MPa with a H2/CO2
ratio of 3. The redution of Cu species was supported by
temperature programmed reduction (TPR) with hydrogen
(H2-TPR) (Figure S6), which exhibited a broad peak in the
o
temperature range of 130-250 C that is consistent with
the reduction of Cu(II) to Cu(0) in the presence of SMSIs.16
X-ray photoelectron spectroscopy (XPS) demonstrated that
CuZn@UiO-bpy contained entirely Cu(0) and a mixture of
Zn(II) and Zn(0) (see below). As a result, we formulated
the active catalyst as ultrasmall Cu/ZnOx NPs encapsulated
in the cavities of UiO-bpy.
2. Catalytic CO2 hydrogenation to Methanol
2.1. Catalytic performace of CuZn@UiO-bpy
CuZn@UiO-bpy was tested for CO2 hydrogenation at
o
250 C and 4MPa with a H2/CO2 ratio of 3. CuZn@UiO-bpy
exhibited a space time yield to MeOH (STYMeOH) of 2.59
-1
gMeOH kgCu h-1 at a gas hourly space velocity (GHSV) of
18000 h-1, greatly exceeding the STYMeOH value of 0.83 gMeOH
kgCu-1 h-1 for the commercial ternary Cu/ZnO/Al2O3 catalyst
in a 6/3/1 ratio under identical conditions (Table 1, S2).
More strikingly, CuZn@UiO-bpy gave a 100% selectivity
o
for methanol at 200-250 C with a GHSV of 18000 h-1,
whereas the ternary Cu/ZnO/Al2O3 catalyst gave a
selectivity of only 54.8 % for methanol at 250oC, owing to
the dominance of the RWGS reaction catalyzed by phase-
separated Cu NPs. Knowing that low CO2 conversions (<
3%) usually give high methanol selectivity18, we also
evaluated the catalytic performance at decreased GHSV
values and higher CO2 conversions. At a GHSV of 1600 h-1,
we obtained a CO2 conversion of 17.4 % and a high MeOH
selectivity of 85.6 % for CuZn@UiO-bpy (Figure S12).
Figure 2. TEM images, HAADF images, and EDX mapping of
Zn@UiO-bpy-Cu (a, d) and CuZn@UiO-bpy (b, e). Cu distribu-
tion is shown in orange whereas Zn distribution is shown in
green. TEM-EDX mapping studies indicated that Cu and Zn
were well-dispersed and mixed throughout the MOF particle.
c) HAADF image of ultrasmall Cu/ZnOx NPs obtained from
digesting CuZn@UiO-bpy with a K3PO4 solution in the pres-
ence of PVP to stabilize the NPs.
Diffuse reflectance UV−Vis-NIR spectroscopy revealed
the formation of Cu NPs after in situ reduction, based on
the disappearance of the MLCT and d-d transition bands of
the bpy-Cu2+ complex and appearance of surface plasmon
resonance (SPR) band of Cu NPs at 573 nm (Figure 1b).
The position of this SPR peak is consistent with a particle
size of less than 1 nm (<30 Cu atoms).17 These NPs can be
confined in the octahedral and tetrahedral cavities inside
the MOFs, which are 1.6 nm and 0.75 nm in diameter
Figure 3. (a) STY of MeOH vs reaction time over a period of
100 h on stream. (b) Selectivity of product vs reaction time.
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The PXRD pattern of the catalyst after reaction was We also prepared CuZn@UiO-bpy catalysts with
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the same as that before the reaction, confirming the
preservation of the MOF structure during catalysis (Figure
1e). Consistent with this, TEM and SEM images of the
samples after the reaction retained the original shape of
MOF particles, and EDX mapping showed even distribution
of Cu and Zn throughout the MOF particles (Fig. 2b and 2e).
different Zn/Cu ratios of 0.26, 0.54, 0.85 and 0.91. As the
Zn loading increased, the STYMeOH increased from 1.05
-1
gMeOH kgCu h-1 for the catalyst with Zn/Cu=0.26 to 2.61
-1
g
MeOH kgCu h-1 for the catalyst with Zn/Cu=0.91 at a GHSV
of 18000 h-1 . Methanol selectivies were 100% for all of
these catalysts (Table S3). These results confirm the
critical role of ZnOx in CO2 hydrogenation.
The catalytic stability of CuZn@UiO-bpy was tested by
running the CO2 hydrogenation reaction for 100 hours. The
selectivity remained unchanged at 100 % for methanol
throughout the experiment, and the activity only slightly
decreased by less than 10 % during 100 h on stream
(Figure 3). In comparison, the commercial Cu/Zn/Al
catalyst lost half of its activity in the first 15 hours. The
selectivity for methanol dropped from 62.7 % to 44.3 %
while the the CO selectivity increased from 37.3 % to 55.7
% at the same time.
2.3. In situ vs. ex situ generated Cu/ZnOx NPs
9
We also prepared a control catalyst with ex situ
generated Cu/ZnOx NPs (DSM-CuZn@UiO-bpy) by a double
solvent method. Concentrated aqueous solutions of ZnCl2
and CuCl2 were added dropwise to a suspension of UiO-bpy
in hexane under vigorous stirring.20 The salts were
absorbed into the MOF cavity due to favourable
hydrophilic interactions and then reduced to NPs by
superhydride (Figure S17-19). This sample exhibited a
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-1
much lower STYMeOH of 0.58 gMeOH kgCu h-1 as compared to
that of in situ generated NPs in CuZn@UiO-bpy (Table 1,
Figure S20), possibly due to less efficient mixing of Cu and
Zn. The SPR peak in the diffuse reflectance spectra moved
to 590 nm, suggesting larger size of the NPs.
Table 1. Catalytic performances of CuZn@UiO-bpy and
control catalysts in CO2 hydrogenation[a]
Catalysts
Cu
wt %
GHSV
CO2
conv.
(%)
Select. (%)
STY
(h-1)[b]
(gMeOH
2.4. SMSIs from the MOF support
kgCu-1 h-1)
MeOH CO
We hypothesized that both bpydc ligands and Zr6
SBUs played important roles in the formation of ultrasmall
Cu/ZnOx NPs via SMSIs. To test this hypothesis, we used
the double solvent method to load Cu and Zn into UiO-67
to form DSM-CuZn@UiO-67 (Figure S21). As expected,
without bpy moieties to provide SMSI, this catalyst was
much less selective for MeOH when compared to DSM-
CuZn@UiO-
bpy
6.9
18000
6000
1600
18000
1600
1600
3.3
100
100
85.6
54.8
51.9
94.6
0
2.59
1.97
1.12
0.83
0.13
0.58
7.2
0
17.4
11.1
5.6
14.4
45.2
48.1
5.4
Cu/ZnO/Al2O3
Cu@UiO-bpy[c]
38.8
11.0
5.8
-1
CuZn@UiO-bpy, with a STYMeOH of 0.40 gMeOH kgCu h-1
(Table 1). DSM-CuZn@UiO-67 was also not very stable
under reaction condition and lost 17 % of its initial activity
over a 25 h reaction period together with a decrease in
MeOH selectivity from 83% to 70% (Figure S22).
DSM-
7.1
CuZn@UiO-
bpy[d]
To probe the role of Zr6 SBUs and confinement effects
of MOF cavities, we changed the Zr-MOF to an Al-based
DSM-
7.2
1600
1600
8.9
8.7
70.8
93.2
29.2
6.8
0.40
0.27
CuZn@UiO-
67[d]
MOF-253,
Al(OH)(bpydc),
constructed
from
{Al(OH)(CO2)}n chains and bpydc ligands (Figure S23-
24).21 The NP/SBU interactions and channel/cavity
structures are vastly different between these two MOFs.
While UiO-bpy possesses octahedral cages with relatively
narrow windows suitable for particle confinement, MOF-
253 exhibits rhombic 1-D channels of 1.2 nm in diameter.
Without strong NP/SBU interactions and/or steric
hindrance of cavity with small windows in MOF-253, the
Cu NPs aggregate to larger nanoparticles of 8-10 nm in
diameter after the reaction and possibly migrate to the
external surface of the MOFs (Figure S25), resulting in an
STYMeOH only one third of that of CuZn@UiO-bpy (Figure
S26-27). The Cu NPs in this size range also showed strong
diffraction peaks in the PXRD patterns (Figure S24).
CuZn@MOF-
253(Al)
14.9
[a] Reaction conditions: H2/CO2 = 3, P = 4 MPa, Time = 30 h, F = 30 mL/min,
Temp = 250 oC. [b] GHSV = 18000 h-1, m = 0.1 g; GHSV = 6000 h-1, m = 0.3
g; GHSV = 1600 h-1, m = 0.3 g, F = 8 mL/min. [c] Cu@UiO-bpy was obtained
by treating UiO-bpy-Cu with NaBEt3H before in situ reduction. [d] The sam-
ples were synthesized via a double solvent method (DSM) with NaBEt3H as
reductant.
2.2 Influence of zinc species on catalytic activity
The critical role of Zn in catalysis was revealed by the
much lower activity of Cu@UiO-bpy, a sample modified
with Cu but not with Zn (Figure S13-15). Cu@UiO-bpy
exhibited a STYMeOH of 0.13 gMeOH kgCu h-1 with a low
-1
methanol selectivity of 51.9 % (for a 30 h run with GHSV of
1600 h-1). Additionally, both STYMeOH and methanol
selectivity quickly dropped over 30 hours, from 0.30 to
3. XPS studies of Cu/ZnOx species and MOF SBUs
The high percentages of Cu/Zn/Zr atoms (>50%)
around Cu/ZnOx and Cu/Zr6 SBU interfaces in the
CuZn@UiO-bpy provide an opportunity to examine the
chemical compositions of these interfaces, which
constitutes only a low percentage of atoms (<0.1%) in
commercial catalysts. We first performed X-ray
-1
0.13 gMeOH kgCu h-1 and from 73.3 to 51.9 %, respectively
(Table 1, Figure S16). The diffuse reflectance spectra of
Cu@UiO-bpy showed an SPR peak around 582 nm,
suggesting the formation of Cu NPs in Cu@UiO-bpy (Figure
1c).19
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photoelectron spectroscopy (XPS) studies on Zn@UiO-bpy-
ZnOx NPs or because of Zn alloying into metallic Cu NPs.24
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Cu (Figure 4a). Before the introduction of reaction gas, Cu
existed mostly in the +2 oxidation state (85.4 %; Cu 2p1/2
and 2p3/2 peaks at 953.4 eV and 933.6 eV) with only a
small amount of Cu(0) species (15 %; Cu 2p1/2 and 2p3/2
peaks at 950.7 eV and 930.8 eV). The Cu(0) species likely
formed as a result of metathesis recation between
(bpydc)CuCl2 and ZnEt2 to form (bpydc)CuEt2, which
undergoes reductive elimination to form (bpydc)Cu(THF)2.
We detected the formation of butane upon treating UiO-
bpy-Cu with ZnEt2 (Figure S28).
The Zn(0) Auger peak is more pronounced in the XPS
spectra of CuZn@UiO-bpy than those reported in the
literature22 due to a high degree of mixing of Cu and Zn and
the formation of ultrasmall NPs. Finally, the XPS data
showed a Cu/Zn ratio of 0.86, which is consistent with the
ICP-OES results.
The binding energy of Zr band was also measured
(Figure 4d). The Zr 3d5/2 and 3d3/2 peaks of Zn@UiO-bpy-
Cu exhibited binding energies of 182.4 eV and 184.9 eV
respectively, corresponding to the Zr(IV) oxidation state in
the [Zr6O4(OH)4(CO2)12] SBU.8a, 25 After treatment with the
reaction gas, the Zr 3d5/2 band split into two peaks at 182.4
eV (52 %) and 181.6 eV (48 %), and the Zr 3d3/2 band split
into two peaks at 184.9 eV (41 %) and 183.8 eV (59 %),
suggesting partial reduction of Zr(IV) to Zr(III).8a This
reduction is likely caused by the spillover of dissociated
hydrogen on the Cu surface to the Zr based SBU.
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4. Relationship between chemisorption of CO2 and H2
and local structures in Cu/ZnOx@MOF
Temperature programmed desorption (TPD) of
H2. The presence of Zr and Zn in lower oxidation states has
also been supported by temperature programmed
desorption (TPD) profiles of H2 (Figure 5). The desorption
peak of H2 on metallic Cu sites was not observed because
o
this process takes place at ~50 C which was out of the
range of our instrument.26 The temperature in the TPD
measurement increased from 100 oC to 250 oC at a rate of 5
o
oC/min, and then was held at 250 C for 60 min. The H2-
Figure 4. XPS data of Zn@UiO-bpy-Cu before and after
introduction of H2/CO2 =3 at 1 atm and 250 oC: a) Cu 2p
region; b) Zn 2p region; c) Zn L3M4.5M4.5 Auger peaks; d) Zr 3d
region.
TPD curve of CuZn@UiO-bpy was deconvoluted into three
broad peaks centered at 194, 243, and 250 °C. The first two
peaks at 194 and 243 °C were assigned as desorption of
hydrogen spilled over onto the Zr-oxide cluster, since they
were also present in H2-TPD of Cu@UiO-bpy but not in that
of UiO-bpy. The hydrogen spillover onto zirconia is well
documented in the literature, and is responsible for the
reduction of Zr(IV) to Zr(III).27 The third peak in the H2-
TPD of CuZn@UiO-bpy was associated with hydrogen
desorption from ZnOx28, as it was also present in the H2-
The Zn@UiO-bpy-Cu was then treated with reaction
o
gas (H2/CO2=3, 1 bar) at 250 C in a reaction chamber
before transferred again to the XPS chamber through a
load-lock gate for analysis without exposure to air. Cu was
completely reduced to Cu(0) in the CuZn@UiO-bpy catalyst
after the treatment (Figure 4a). Notably, Cu was only
partially reduced to Cu(0) in Cu@UiO-bpy after the same
reduction procedure (Figure S29). When compared to bulk
Cu(0) with Cu 2p1/2 at 952.5 eV, the Cu/ZnOx NPs showed
lower Cu binding energy (Cu 2p1/2 at 951.4 eV), probably
as a result of electron injection from the conduction band
of ZnOx to Cu.22
TPD of CuZn@MOF-253(Al).
Furthemore, it was
previsously shown that Al2O3 does not absorb spillover
hydrogen.16
The chemical nature of Zn plays an essential role in
CO2 hydrogenation. The Auger Zn L3M4.5M4.5 line serves as
a sensitive measurement of the oxidation status of Zn. In
XPS spectra of CuZn@UiO-bpy after reacting with the
reaction gas flow (1atm, H2/CO2=3) at 250 oC, the Zn Auger
region consisted of two sets of lines at 498 eV (67%) and
495 eV (33 %), attributable to Zn(II) and Zn(0),
respectively (Figure 4). In comparison, pure ZnO tablet and
Zn@UiO-67, the UiO-67 treated with ZnEt2 but without Cu
loading, only showed one line of Zn(II) at 498 eV after
treatment with reaction gas (Figure S30) while the
commercial tertiary Cu/ZnO/Al2O3 catalyst contained only
a small amount of Zn(0) (10 %) under the same
conditions.23 The presence of Zn(0) species in CuZn@UiO-
bpy is either due to rich oxygen vacancies in the ultrasmall
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o
Figure 5. H2-TPD profiles from 100 to 250 C. The green and
yellow peaks correspond to H2 desorption on Zr sites and the
cyan peaks are attributed to H2 desorption on ZnOx.
Figure 6. CO2-TPD profiles of various MOF samples in the 50 -
250 oC temperature range.
TPD of CO2. Chemisorption of CO2 on the catalysts
was investigated by CO2-TPD. The temperature was
ramped from 100 C to 250 C at a rate of 5 C/min, and
then held at 250 C for 60 min. The CO2-TPD profile of
o
The peak at around 150 C in the CO2-TPD profile
might be related to bpy site, since it is also present in the
profile of UiO-bpy but not in that of UiO-67. The peak
around 240 oC can be attributed to Cu NPs since it was also
present in the profile of Cu@UiO-bpy. The presence of
these multiple sites for CO2 adsorption was also verified by
in situ DRIFTS. The multiple negative peaks in the 1400-
1750 cm-1 region correspond to the carbonate and
bicarbonate species absorbed on Cu/ZnOx or Zr6 SBU.
(Figure S31-32).
o
o
o
o
CuZn@UiO-bpy was deconvoluted into four peaks at
184°C, 247 °C and holding at 250 °C for 3.5 min and 12 min
o
(Figure 6). The two strong peaks at >250 C also appeared
in the CO2-TPD profiles of other UiO-MOF-based samples,
even without Cu/Zn metalation, suggesting that they are
due to CO2 adsorption on unsaturated Zr sites on the SBUs.
Vacant sites on the Zr-SBUs of UiO MOFs can be generated
at relatively low temperatures at 200-250 C (Figure 7).
The 9.1 % capping formates on the SBUs can decompose
o
o
above 200 C and leave two adjacent 7-coordinated Zr(IV)
sites. In fact, even without formate caps, the Zr6-cluster
(SBU) is reported to lose two µ3-OH and two protons at
250 oC, leaving six 7-coordianted Zr(IV) sites.14 These open
sites can adsorb CO2 and accept dissociated hydrogen to
form Zr-H, likely playing a key role in catalytic CO2
hydrogenation.
Figure 7. Formation of unsaturated Zr sites that can accept
CO2 and hydrogen spillovered from Cu surfaces.
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DRIFT spectra. This material is available free of charge via the
Internet at http://pubs.acs.org.
■ DISCUSSION
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AUTHOR INFORMATION
Corresponding Author
wangchengxmu@xmu.edu.cn
wenbinlin@uchicago.edu
8
Notes
9
These authors declare no competing financial interest.
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ACKNOWLEDGMENT
We thank Zi Wang and Ruihan Dai for experimental help. We
acknowledge funding support from the National Natural Sci-
ence Foundation and Ministry of Science and Technology of
the P. R. China (NSFC21471126, NSFC21671162,
2016YFA0200702), the National Thousand Talents Program
of the P. R. China, the 985 Program of Chemistry and Chemical
Engineering Disciplines of Xiamen University, and the U.S.
National Science Foundation (DMR-1308229)
Figure 8. Schematic showing the encapsulated active sites in
MOFs and the functions of the various surface sites in catalytic
CO2 hydrogenation.
Our experimental findings indicate that rich
surface/interface sites on well-mixed Cu, ZnOx, and Zr6
SBUs contribute to the adsorption and activation of H2 and
CO2 in the CuZn@UiO-bpy catalyst (Figure 8). Based on
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established
mechanistic
understanding
of
the
Cu/ZnO/Al2O3 catalyst, we propose that H2 first undergoes
homolytic dissociation on the Cu surface to form Cu-H
species. The dissociated hydrogen spillovers to Zr sites on
the SBUs and defect sites on ZnOx. Meanwhile, CO2 is
adsorbed on unsaturated Zr sites and ZnOx to form
carbonates and bicarbonates, which are quickly
hydrogenated by the activated hydrogen from spillover.
The spillover hydrogen can also reduce some Zr(IV) to
Zr(III) and some Zn(II) to Zn(0), as observed by XPS. The
synergystic combination of hydrogen activation sites on Cu
and CO2 activation sites on ZnOx and Zr SBUs leads to
superior performance of the CuZn@UiO-bpy catalyst.
■ CONCLUSION
We have generated ultrasmall Cu/ZnOx nanoparticles
in MOF cavities in situ under CO2 hydrogenation
conditions. The strong interactions between Cu/ZnOx
nanoparticles and the bpy moieties on the ligands as well
as Zr6 SBUs stabilize these ultrasmall well-mixed
nanoparticles and prevent the agglomeration of Cu NPs
and phase separation between Cu and ZnOx. A high degree
of mixing between Cu and ZnOx leads to formation of low
valence Zn and Zr in the presence of H2 at 250 oC, affording
high catalytic activity and selectivity for methanol
synthesis from CO2 hydrogenation. Our work highlights
new opportunities in using MOFs as novel supports for
metal nanoparticle catalysts by taking advantage of
tunable and specific strong metal-support interactions.
ASSOCIATED CONTENT
Supporting Information.
General experimental; procedures for catalyst characteriza-
tion; catalyst preparation and evaluation of catalytic perfor-
mance, including additional catalytic data and TEM images of
catalysts before and after reactions; XPS analysis and in situ
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